The aim of this project is to describe the structure of tobacco mosaic virus in molecular detail, and to use this structure and related structures to determine the mechanism of assembly of TMV. X-ray diffraction pictures of oriented gels of TMV are being used, as well as diffraction pictures from six heavy atom derivatives. Diffraction from the native virus is usable to 3 A resolution. Diffraction from the derivatives is usable to 3.5 A or better. It is proposed to collect data from two or three more derivatives, one of which is already confirmed as usable, and calculate an electron density map at 3.5 A resolution. The current map is at 4 A resolution. From this map, a molecular model will be built. If possible, the parameters of this model will be refined against the 3 A native data. The TMV protein helix, the lead derivative and the U2 strain of TMV will also be studied. These structures all involve modifications in the carboxyl-containing region of the protein near the RNA which is believed to be the control site for assembly.